[WIP] animations

[Rich-Harris]

see #1431

[codecov-io]

Codecov Report

Merging #1454 into master will increase coverage by <.01%.
The diff coverage is 93.18%.

@@            Coverage Diff            @@
##           master   #1454      +/-   ##
=========================================
+ Coverage    91.1%   91.1%   +<.01%     
=========================================
  Files         122     124       +2     
  Lines        4521    4555      +34     
  Branches     1422    1434      +12     
=========================================
+ Hits         4119    4150      +31     
- Misses        159     160       +1     
- Partials      243     245       +2

Impacted Files	Coverage Δ
src/validate/js/propValidators/index.ts	100% <ø> (ø)	⬆️
src/compile/dom/Block.ts	93.75% <100%> (+0.29%)	⬆️
src/compile/Compiler.ts	96.06% <100%> (+0.05%)	⬆️
src/validate/js/index.ts	82.85% <100%> (ø)	⬆️
src/validate/index.ts	90% <100%> (+0.2%)	⬆️
src/parse/read/directives.ts	80.95% <100%> (+0.3%)	⬆️
src/validate/html/validateElement.ts	86.99% <100%> (+1.4%)	⬆️
src/compile/nodes/Element.ts	93.33% <100%> (ø)	⬆️
src/compile/nodes/EachBlock.ts	98.11% <100%> (+0.02%)	⬆️
src/validate/js/propValidators/animations.ts	60% <60%> (ø)
... and 3 more

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[Rich-Harris]

It basically works:

I'm unhappy with the state of the code; I tried to refactor all the transition code earlier today in preparation for this work, but couldn't quite untangle it. We can take another run at it another time.

This isn't quite done — it doesn't like it if you mash the button rapidly, and it doesn't seem to play well with transitions — but I'm inclined to think we should release this and treat those things as follow-up bugs.